configure.in: Rename to configure.ac.

	* configure.in: Rename to configure.ac.
	* configure.ac: Renamed from configure.in; make minimum necessary
	changes for autoconf 2.5x.
	* aclocal.m4: Make minimum necessary changes for autoconf 2.5x.
	* configure: Regenerate with autoconf 2.57.

From-SVN: r75362